Self-adhesives are used as processing assistants in many technical fields, allowing different components of a system to be joined with one another in a simple way. The term “self-adhesive” encompasses pressure-sensitive adhesives and hotmelt adhesives, in other words all adhesives which inherently allow adhesive bonding to the substrate in question. These self-adhesives may be used in different ways. For reasons of ease of handling, self-adhesives are employed typically in the form of coatings of self-adhesive that are part of a self-adhesive sheetlike element, in the form, for example, of a self-adhesive tape in roll form, or in the form of a thin, extruded self-adhesive film, which can be obtained without using solvents. Coatings of self-adhesive, furthermore, can also be produced when the self-adhesive is applied as a liquid solution or dispersion in a coating step or printing operation.
Particularly for use in electronic devices, the requirements imposed on self-adhesives are very high. As well as low outgassing, the self-adhesives ought to be able to be used within a wide temperature range, and ought to have particular optical and electrical properties. In view of the increasing miniaturization of electronics, a fundamental objective with coatings of adhesive is to realize further functions within the layer of adhesive, in addition to the adhesive bonding capacity, in order thereby to allow savings to be made in terms of operating steps during the manufacture of the devices. By this means, furthermore, it is possible to dispense with additional functional elements, thereby reducing the size and weight of the assembly in question.
Highly functionalized self-adhesives and self-adhesive tapes are used in different electronic and electrical components. In rigid or flexible miniaturized electronic circuits, for instance, they allow signal coupling or decoupling of components and their component parts; as a result of their high flexibility, the self-adhesives exhibit a high long-term stability, even under mechanical deformation, and at the same time are easy to process, and can therefore be used even on irregularly shaped surfaces.
Self-adhesives and self-adhesive tapes are used more particularly for the bonding of display systems, and in the case, for example, of liquid-crystal displays (LCDs) with additional functional layers, or of touch-sensitive displays (touch panels) in cell phones, computer screens, televisions, and compact computers such as notebooks, laptops or personal digital assistants (PDAs).
In recent years, in view of the significantly lower energy consumption, light sources based on luminescent emitters have increased in importance for these display devices. In contrast to thermal emitters, such as incandescent lamps, for example, the light emitted in light sources of this kind is generated as part of luminescence processes which occur within a phosphor. Luminescence is the term for a process in which an excited phosphor undergoes transition, with emission of electromagnetic radiation, to a state of lower energy (radiative deactivation). In practice, systems that are of interest as luminescent emitters are more particularly those where the excitation of the phosphor takes place in an electrical field. The accompanying emission of light and other electromagnetic radiation is referred to as electroluminescence. Another technical utilization of electroluminescence besides light-emitting diodes (LEDs) is that of what are called capacitor-type luminous films. With the capacitor-type luminous films (electroluminescent or EL film, luminescent sheet, luminous film), the phosphor is located between two electrodes in a special capacitor arrangement. The phosphor is excited by application of an electrical alternating field to the electrodes.
For capacitor-type luminous films as well it is possible to use self-adhesives which have a multiple-functionality setup. For example, DE 10 2006 013 834 A1 discloses a pressure-sensitive adhesive which is used as a binder matrix for electroluminescent phosphor. As examples of pressure-sensitive adhesives which can be used in principle, said document gives a general recitation of different adhesive systems, as for instance those based on poly(meth)acrylates, silicones, polysiloxanes, synthetic rubbers, polyurethanes, and also adhesive systems based on block copolymer.
DE 10 2006 013 834 A1 is particularly comprehensive in its description of poly(meth)acrylate-based self-adhesives, i.e., self-adhesives which comprise as their base polymer a polymer formed from a (meth)acrylate monomer mixture (the term “(meth)acrylate” serves below for a simplified description and includes not only acrylates but also methacrylates, in other words their acids and their derivatives, esters, for example). Said document cites, more particularly, mixtures which, at 70% to 100% by weight of the monomer mixture, comprise one or more (meth)acrylic esters esterified with alkyl alcohols having 1 to 30 C atoms, and which, furthermore, may optionally have not more than 30% by weight of one or more olefinically unsaturated monomers with functional groups. In practice it has been found that, where these self-adhesive systems are used with commercial phosphors, electroluminescent devices of good luminous power and long-term stability can be produced.
It is an object of the present invention to provide a base polymer for a self-adhesive which, as a binder matrix in an electroluminescent device, offers an even greater luminous power and also an improved long-term stability by comparison with polymers known from the prior art.